Electrical amplifier circuit



April 24, 1928.

L. M. HULL.

ELECTRICAL AMPLIFIER CIRCUIT Filed Aug. 24, 1927 Z r V e z z m 1 w s 2 7 Z y Y an .m 4 a a V m? a u m M I 3mm;

Patented Apr. 24, 1928.

UNITED, STATES PATENT OFFICE.

LEWIS HULL, OF BOONTON, NEW JERSEY, ASSIGNOR TO RADIO FREQUENCY LABO- RATORIES, INCORPORATED, F BOONTON, NEW JERSEY A CORPORATION OF NEW JERSEY.

' ELECTRICAL AMPLIFIER CIRCUIT.

\ Application filed August 24, 1927. Serial No. 215,189.

This invention relates to electrical amplifier circuits of the type adapted for use in connection with a vacuum tube. Such a vacuum tube may include an emitting cathode, an anode, and one ormore additional elements having the form of grids or the like. The invention relates especially to a balanced amplifier circuit of theforegoing type which is especially suited for use in a radio receiving set, and which provides means for reducing or eliminating retroaction or regeneration from the output or plate circuit of the amplifier stage into the input or grid circuit thereof. Such retroaction 1 may be caused by coupling due to capacities between the elements oft-he vacuum tube and their associated connections, and especially by coupling due to the natural capacity between the anode and control element or grid of the vacuum tube.

Figure 1 shows a single "amplifier stage embodying my invention. Figure 2 shows an amplifier stage employing a double grid vacuum tube and embodying my invention. Referring to Figure 1, which shows a preferred embodiment of my invention,the input circuit of the amplifier stage may assume any desired form, and is here represented as being constituted by the tuned circuit 'Lo-Co. One terminal of the input circuit is connected to the control electrode or grid G. The other terminal is connected'to the cathode F, which may be any emitting structure, suitably a filament heated by current from. the battery A or from an alternating current source. My invention applies equally, of course, to vacuum tubes having cathodes heated 'by means of alternating current, inwhich case a centre tap or other conventional arrangement may be em? ployed. 'A natural capacity exists between the control elementGr andthe anode or plate P and their associated connections, and this capacity may be represented by the'dotte'd lines at Cm. The plate coil is shown at L,, and may form the primary of an output transformer L,L of which the secondary may suitably be tuned by means of the condenser G,. Connected in series with the plate coil L, between anode P and cathode F,'and coupled thereto, is a balancing coil L The coils L, and L may be coupled together with any desired degree of coupling up to and includingclose or substantially I have found, when the couplingemployed is substantially less than unity coupling-or its equivalent. Any suitable source of space current, for example a B-battery, B, may be included in series with the coils L, and L and may be by-passed by condenser C as shown. Between the control electrode 01' grid G' and the junction point of coils L, and L is connected a balancing condenser 0,, which may suitably be adjusted to the proper value when the amplifier is built, and will probably require no adjustment thereafter. A C-battery may be employed for biasing the grid, if desired. I

The coils L, and L are of course coupled and connected in the proper sense, as shown, so that currents flowing through the balancing capacity C, reduce or substantially balance out undesired currents flowing through the capacity Cm or other naturalexpressed as follows where the several mutual induct-ances are as indicated in the drawing. A subsi(liary balance condltlon, which does not require exact fulfillment in order to obtaingood operation.

Cm M L where L is the tot-alplate inductance, i. e., 'V

of the double grid type is shown in Figure 2, the inner grid Gr being employed as control element, and the outer grid G being given a positive bias. This may be done as shown (by-pass condensers C being used iif necessary), or by means ofa separate batu tery, or 'connectlon to a socket-power device,

or otherwise, The foregoing connection of thegrids Gr and G may be reversed 1n certam cases.

by means of a connection'to the Bf-battery Any number of amplifier stages similar "tothose shown in Fig.1 or inFig. 2 may be cascaded together if desired, or such stages maybe cascaded with unbalanced or untuned stages, in a well recognized manner. -A preceding stage may suitably be coupled through coil L-to the coil Lo of the tuned circuitLo-Go; a succeeding stage maysuitably be. connected'to the terminals 1 and 2, as by connecting these terminals to the con- "trol element and cathode, respectively, of

the next tube. According'to a preferred embodiment of the invention, therefore, each amplifier circuit or stageis provided with an input transformer L-Lo and an output transformer L a-L as described above and coupled -to said plate coil,

'betweensaid control electrode" and the junctionof said plate coil and said second coil, whereby said amplifier circuit may be balanced to reduce retroaction, i

2. An electrical amplifier circuit comprising, in combination, vacuum tube device including an anode, a cathode, and a control electrode, an input transformer having a secondary connected between said control electrode and said cathode, an output transformer havinga primary connected atone terminal toi'sand anode, a coil coupled to said output transformer primary "and con nected between a second terminal thereof and said cathode, and .a balancing, condenser connected between said -control'electrode and the junction of saidvcoil and said output transformer primary, whereby said amplifier circuit may be balanced to reduce retroaction. f

3. An electrical amplifier circuit comprise ing, in combination, a vacuum tube device including an anode, a cathode, a control .electrode, and a fourth electrode, an nput transformer having a secondary connected I between said control'electrode and said cathode, an output transformerihaving a primary connected at one termmal to said a plate in series therewith betweenisaidcathode and anode, and a balancing condenser connected anode, a coil coupled 'to said output transformer primary and connected between a 3 second terminal thereof and said cat balancing condenser connected between said control electrode I and the junction of said coil and said output-transformer primary,- andmeans for imparting a positive potential to said fourth electrode with respect to sa d cathode, whereby said amplifier circuit may giving 'a relatively high amplification;

ture.

.LEWIS M HULL,

ode, a p

be balanced to reduce retroaction while "In testimony whereof, I afiix my signa'-' 

